For oil wells and gas wells in environments containing carbonic acid gas, it has been common to use 13% Cr martensitic stainless steel pipes excellent in carbonic-acid gas corrosion resistance. However, recent increasing depth of oil wells and gas wells (hereinafter, abbreviated as oil wells) requires materials higher in strength than has hitherto been required. The oil well environment is such that as the depth of the oil well is increased, the environment becomes higher in temperature and pressure, and higher in the partial pressures of carbonic acid gas and hydrogen sulfide. Therefore, steel pipes having sufficient corrosion resistance even in severer environments come to be needed.
Since the corrosiveness of carbonic acid gas at high temperatures is generally controlled by the content of Cr, a composition design for further increasing the content of Cr is required for the purpose of improving the corrosion resistance of a steel pipe. However, when the content of Cr is increased, generally 8-ferrite is produced, and accordingly no martensitic single-phase micro-structure comes to be obtained and the strength and the toughness are deteriorated. Therefore, in oil wells requiring high strength, two-phase stainless steel pipes produced by cold working have been frequently used. However, unfortunately, the two-phase stainless steel pipes contain large amounts of alloying elements and further require a special production step of cold working, and hence the two-phase stainless steel pipes are not such materials that can be offered inexpensively.
Accordingly, recently, there have been investigated steel pipes in which martensitic stainless steel is taken as the base material, and the amount of Cr is further increased as compared to conventional steel pipes. Examples of such investigations include Patent Documents 1 to 16.    Patent Document 1: JP3-75335A    Patent Document 2: JP7-166303A    Patent Document 3: JP9-291344A    Patent Document 4: JP2002-4009A    Patent Document 5: JP2004-107773A    Patent Document 6: JP2005-105357A    Patent Document 7: JP2006-16637A    Patent Document 8: JP2005-336595A    Patent Document 9: JP2005-336599A    Patent Document 10: WO2004/001082    Patent Document 11: JP2006-307287A    Patent Document 12: JP2007-146226A    Patent Document 13: JP2007-332431A    Patent Document 14: JP2007-332442A    Patent Document 15: JP2007-169776A    Patent Document 16: JP10-25549A